(a) Field of the Invention
The present invention relates generally to a method for forming a photoresist pattern, and more particularly, to a method for forming a photoresist pattern capable of improving adhesion strength of the photoresist pattern formed on a substrate.
(b) Description of the Prior Art
Recently, the field of pathology and/or biotechnology related to disease diagnosis or efficacy evaluation of chemotherapy, respectively, has been increasingly studied as future technology. A biochip has been employed as one of the methods of accelerating these studies. Applications considered as a biochip include a cell binding chip or an electrolysis chip.
In the cell binding chip, it is useful to increase a surface area of a microstructure to which a cell is bound in order to increase cell binding efficiency. The electrolysis chip is used to extract deoxyribo nucleic acid (“DNA”) in a cell bound to the microstructure.
If an SU-8 photoresist, commercially available from Microchem Company, is applied as a negative-type photoresist in order to form a microstructure, the microstructure can be formed by a single process of forming a photoresist pattern without additional processes, and thus the SU-8 photoresist is widely used to form various microstructures.
The biochip sometimes requires microstructures having a high aspect ratio on various substrates. For example, a photoresist pattern formed on a silicon substrate using the SU-8 photoresist has an excellent adhesion property on the silicon substrate, and thus it is possible to form the SU-8 microstructures having a high aspect ratio of 10 or more.
However, it is often difficult to steadily maintain the adhesion properties between the microstructure and substrate when various substrates are used. Hence, it is essential to ensure excellent adhesion strength in various substrates in order to form a stable microstructure having a high aspect ratio.
In particular, it is necessary to improve the adhesion property of the microstructure formed on a substrate of glass, metal and metal oxide on which the adhesion strength is found to be weak.
A method of improving the adhesion strength of the photoresist pattern formed on various substrates is disclosed in U.S. Pat. No. 6,741,819 and U.S. Patent Application Publication No. 2004/0214098. The U.S. patent and published patent application show methods including coating a polyimide layer as an adhesion layer before a photoresist layer is formed on a substrate or adding a material to the photoresist solution for improving adhesion strength.
In the case of coating a polyimide layer in order to improve the adhesion property of SU-8 photoresist, the organic polyimide layer can have an influence on the photoresist pattern, or is sometimes not suitable to be used for the existing microstructure. In the case of adding a material to the photoresist solution to improve the adhesion property in the SU-8 photoresist, it is difficult to produce a suitable SU-8 photoresist solution, and thus is not substantially suitable for direct use.
Further, adhesion layers such as a self-assembled monolayer (“SAM”), hexamethyldisilazane (“HMDS”) and OmniCoat (commercially available from Microchem Company) can be applied to improve the adhesion strength of the SU-8 microstructures on the glass substrate.
In spite of forming the photoresist pattern on the glass substrate using these methods, the generated SU-8 microstructures separate from the glass substrate due to insufficient adhesion strength.
FIG. 1 is a top plan view photograph showing the SU-8 microstructures formed on a silicon substrate according to the prior art. FIG. 2 is a photograph showing the SU-8 microstructures formed by a conventional method for forming the SU-8 microstructures on a glass substrate, in which the SU-8 microstructures are stripped off in part.
It can be found from the photograph of FIG. 1 that the SU-8 microstructures formed on the silicon substrate maintain a clear pattern due to excellent adhesion properties. In contrast, in FIG. 2, the SU-8 microstructures formed on the glass substrate are stripped off from the glass substrate due to insufficient adhesion strength, and thus fail to maintain a clear pattern. Further, the same result of the SU-8 microstructures being stripped off without improving adhesion strength was obtained, in spite of applying the above-described method for improving adhesion strength.
Therefore, as these results indicate, it is essential to form the photoresist layer having excellent adhesion on various substrates in order to allow the microstructures formed from the photoresist layer to be effectively used.